I very frequently use the Find command, and am used to using Command-F to get to it! Now, unless I opened LR within the last couple of minutes, I just have it stupidly alternating between full-screen mode!
Others have reported this as well. Please add your vote and details to this bug report in the official Adobe feedback forum: LIGHTROOM - Find and Full Screen command key shortcut conflict under OS X 10.11. Include your version of LR as reported by Help > System Info, your operating system and version, and details about when the problem occurs.